In a combined-cycle power plant having a gas turbine installation and a steam turbine installation, the hot exhaust gas from the gas turbine is fed to a heat recovery steam generator. The heat of this hot exhaust gas is utilized in the heat recovery steam generator to generate steam, which steam is used to operate a steam turbine installation. The heat recovery steam generator and steam turbine installation are in this case operated in a water/steam cycle. To close this cycle, the steam which has been expanded in the steam turbine installation is fed to a condenser, where it is condensed. The condensate is fed back to the heat recovery steam generator in order to be heated again, evaporated and possibly superheated.
The water/steam cycle, which substantially includes the heat recovery steam generator, the steam turbine installation, the condenser, the feedwater vessel/degassing means, as well as the pumps, connecting pipes, etc., is a system in which very high demands are imposed on the purity of the working medium, and a special water-chemical procedure is used to lower the levels of corrosion and deposits.
To make optimum use of the heat of the exhaust gas from the gas turbine, the heat recovery steam generator comprises up to three pressure stages for generating steam at different pressure levels. A pressure stage of this type therefore includes an economizer and evaporator and, depending on the specific installation or the steam generating process, also a steam drum and superheater.
To ensure correct chemical operation of the water/steam cycle, the working medium has to be degassed, which can be carried out, for example, by means of a thermal process. Degassing of the working medium is particularly important if the system was at ambient pressure and had been opened, i.e. after inspection and/or maintenance work or during cold starts. The degassing also requires particular attention during and after periods where large quantities of make-up water are used.
According to the prior art, the thermal degassing may take place either in the condenser, in the feedwater vessel/degassing means, or by means of a degassing means which has been fitted to the low-pressure drum. In particular the latter two variants require high investment costs on account of the additional components required. An additional factor in these variants is that they are disadvantageous in terms of energy on account of the need for heating steam for preheating a liquid which has been slightly supercooled (supercooling 5 to 20 K) to saturation temperature. The higher the pressure level of the degassing means, the greater this drawback becomes.
With regard to the thermal degassing of the working medium, it should be ensured that the use of heating heat for the degassing procedure takes place in accordance with the prevailing qualitative and quantitative requirements, in order to ensure that the overall efficiency of the installation is reduced as little as possible and only for a limited time.
If the systems are functioning perfectly and if the installations operating in the subatmospheric pressure range are properly sealed, it is not absolutely inevitable that degassing will be required throughout the entire operating time of the installation. For this reason, it is also not sensible to ensure permanent high-quality degassing with high investment costs and a constant power loss. Rather, it is more appropriate to use additional measures to ensure appropriate degassing in the event of special conditions or when time-limited procedures are being implemented, but to operate the installation without this additional degassing during normal operation.
With regard to the degassing in the feedwater vessel which is of relevance in the context of the present invention, reference is made in particular to EP 0 826 096, which describes a process and an apparatus for degassing a condensate. This document proposes that a condensate which has already been preheated but has been mixed with cold condensate be fed to the feedwater vessel for degassing, and that a partial stream of the same condensate, which has likewise been preheated but not mixed with cold condensate, be used as heating medium for the heating for the purposes of degassing. In this case, the condensate is preheated in a condensate preheater which is arranged in the exhaust-gas-side outlet region of the heat recovery steam generator. The condensate which has been degassed in this manner in the feedwater vessel can then be fed to the pressure stages of the heat recovery steam generator.
One drawback of this process in terms of energy is that a partial stream of the condensate which has already been preheated has to be cooled again by admixing cold condensate.